Peer authentication is a method by which a first user is authenticated by a second user. (Note that, as in the term peer-to-peer communication, the word “peer” is used generically and has no connotation regarding the professional or social standing of the users.) An example of peer authentication is illustrated by the following familiar scenario: an employee in the lobby of a corporate building realizes that she accidentally left her corporate badge at home, and that therefore she will not be allowed to enter the building proper without some other means of authentication. She therefore approaches the guard in the lobby and tells the guard that she is an employee, but that she doesn't have her badge.
The guard then:                asks the employee for her name;        looks up the name in a computer database;        notes the employee's office number;        submits a query to the database to determine who the employee's officemate is; and        calls the officemate, asking him to come to the lobby to ID the alleged employee.        
The officemate arrives at the lobby, and then either:                verifies that the alleged employee is indeed an employee of the company, or        tells the guard that he does not recognize the alleged employee.In the former case, the guard permits both employees to advance past the lobby, while in the latter case, the guard stops the alleged employee from advancing past the lobby, and perhaps takes some additional action (e.g., calls the police, etc.).        
In a variation of the above scenario, the guard, rather than asking the officemate to come to the lobby, might ask the officemate to talk to the alleged employee over the phone. The officemate talks to the alleged employee over the phone, and determines whether the alleged employee is in fact who she claims to be, based on the telephone conversation (e.g., based on her voice, based on her answers to one or more questions, etc.). The officemate then informs the guard whether the alleged employee should be allowed to advance past the lobby.